Frictional heat is generated at the interface of two elements in a relative motion as a product of surface rubbing. Some of the heat generated is dissipated through the non-contacting surface area into a convective medium, such as air or a lubricant. However, a considerably large amount of heat is conducted into the solid media of the contacting elements, resulting in a temperature increase in the solid. The temperature of the contact interface can be much higher than that of the surrounding material. Today, the demands for compact design may require bearing surfaces to work under severe conditions, such as high loads. It is anticipated that the accumulation of frictional heat will become a major problem in these situations.
Bearings with less lubricant supply are attractive due to the economic and environmental considerations. Lubrication by solid powder represents a new direction in bearing development (ASME 95-Trib-42, by Heshmat and Brewe, 1995, pages 1 to 2). However, heat is a serious barrier that controls the load capacity of the bearing and limits the reduction in the use of fluid lubricants. Solid lubricants can replace the lubrication function of the conventional fluid lubricants. It is anticipated that if the cooling function is provided by appropriate means, bearings with solid lubricants can compete with the fluid-lubricated journal bearings.
Due to the continuous release of the frictional heat, the contact interface constantly works under a high temperature, facing scuffing and seizure failures (Wang et al., Tribology Transactions, pp. 587-593, 1994; Ni and Cheng, Tribology Transactions, pp. 121-129, 1995). Temperature uniformity is critical for conformal contact elements, such as journal bearings, whose performance is strongly controlled by clearance. Due to the bearing structural distortion caused by frictional heating, the bearing may lose its designed clearance, resulting in multi-contacts and seizure failure (Hazlett and Khonsari, Tribology International, pp. 177-182, 1992; and Wang et al., ASME 95-Trib-33, 1995). It is clear that temperature reduction at the contact interface and prompt dissipation of frictional heat from the contact region are crucial to protecting tribological interfaces and retaining the anti-failure capability of bearing surfaces. They are also critical to the development of bearings for future machinery with high load capacity and less lubricant consumption.
Generation of frictional heat at the contact interface creates heat sources. If a heat sink could be created at appropriate locations, it would be possible to transfer the frictional heat directly away through the sink. Heat pipes are ideal heat sinks for this consideration. The isothermal journal bearings that incorporate heat pipes can be ideal structures for bearings with low tendency of failure. These bearings may find wide applications in heavy-duty machinery. They can also be used for the bearings lubricated by a solid lubricant, such as lubricious powders, or as bearings without lubrication.